The Time Lords' energy is being drained through a black hole, so all three
Doctors are summoned to investigate. While the First Doctor advises them
from the TARDIS scanner, the Second and Third Doctors travel through the
black hole. They discover that Omega, a long-lost pioneer of Time Lord
history, is imprisoned therein. Believing he has been abandoned, Omega --
now wielding supreme power over the anti-matter universe within the black
hole -- is determined to destroy the Time Lords, and force the Doctors to
remain within the black hole in his place.

Production

For each of the first three seasons in which Jon Pertwee played the
Doctor, Doctor Who had benefitted from strong “hooks”
in each year's leadoff story: Pertwee's debut in Season Seven's Spearhead From Space, the introduction of the
Master in Season Eight's Terror Of The
Autons, and the return of the Daleks in Season Nine's Day Of The Daleks. Producer Barry Letts wanted
to continue this tradition for the programme's tenth season, and felt that
something to celebrate the show's anniversary was appropriate.

For quite some time, Letts had been encouraged -- both by fans and by
others at the BBC -- to devise an adventure in which Pertwee's Third
Doctor met his two predecessors, played by Patrick Troughton and William
Hartnell. To date, Letts had dismissed this as being too fannish a notion,
but in early 1972 he and script editor Terrance Dicks decided that a
multi-Doctor story would be an excellent way in which to celebrate
Doctor Who's decade on the air.

Originally, the Time Lords were in conflict with a
Federation of Evil led by a personification of Death

Dicks discussed the idea with Bob Baker and Dave Martin as the pair were
completing work on The Mutants for Season
Nine. On February 3rd, Baker and Martin submitted an idea called
“Deathworld”. In it, the Time Lords are in conflict with a
Federation of Evil led by a personification of Death. To avert all-out
war, the Time Lords manage to convince the Federation to allow them to
send the three Doctors into the Federation's Underworld domain. There, the
Doctors will do battle against various realisations of Death -- including
zombies, the Four Horsemen of the Apocalypse, the Hindu goddess Kali, and
the cyclops Polyphemus from Greek mythology -- with the victor in the
contests determining whether the Time Lords or the Federation of Evil will
prevail.

Meanwhile, the Doctor Who production office contacted both Hartnell
and Troughton to confirm their interest in returning to the series.
Despite having been critical in the press of the programme's latter-day
direction, Hartnell -- who had virtually retired from acting due to the
debilitating arteriosclerosis which had been part of the reason he left
Doctor Who in 1966 -- was quick to accept the offer. Troughton also
agreed, but his schedule was so busy that he would not be available until
November.

Consequently, it was decided that the anniversary story would be made
second in the tenth recording block, making it the third Season Ten story
to be taped after Carnival Of Monsters
(completed as part of the ninth production block) and Frontier In Space. “Deathworld”
therefore became Serial RRR. Pertwee was also approached to ensure he
approved of the return of his predecessors, and gave his consent with the
condition that his Doctor remain the adventure's focus.

Baker and Martin continued to work on their storyline during the spring of
1972. The main enemy became Ohm -- so named because “OHM” read
upside-down is “WHO”, reflecting the writers' desire for him
to be seen as the Doctor's opposite number. By mid-April, it became clear
to the production team that Hartnell's health was too poor for him to take
an active role in the entire serial. The First Doctor's involvement in the
storyline would therefore have to be restricted to brief scenes which
could be prefilmed. Dicks also asked Baker and Martin to include former
companion Jamie McCrimmon (played by Frazer Hines) in the adventure, and
suggested a romantic subplot involving Jamie and Jo Grant.

Due to William Hartnell's health, the First Doctor would
be restricted to brief, prefilmed scenes

As the spring wore into summer, Letts and Dicks asked the Bristol Boys to
make further changes to the storyline. Letts disliked the name Ohm (since
the Doctor's real name was not “Who”) and so the central
villain became Omega, named after the final letter of the Greek alphabet.
Letts also felt that the “deathworld” setting was
inappropriate for Doctor Who, believing that the limbo-like
Underworld would be too incredible for the viewing audience. The setting
was therefore changed to a planet on the other side of a black hole, and
the monstrous Gell Guards (never named onscreen) were introduced. The
scripts were finally commissioned under the title The Three Doctors
on June 27th.

Baker and Martin rearranged Hartnell's role so that the First Doctor
featured heavily only in episode four, when he joined his two successors
for a final confrontation with Omega. After consulting with Hartnell's
wife, Heather, however, Letts and Dicks realised that even this limited
schedule would be too arduous for the actor, whose memory was becoming
increasingly unreliable. Dicks therefore rewrote the scripts so that the
First Doctor appeared only on the TARDIS scanner -- material which could
be filmed in a single day. Hartnell and Troughton were both issued
contracts on September 21st.

The story's title was changed to “The Black Hole” around the
time that its director, Lennie Mayne, came on board. Mayne had helmed The Curse Of Peladon the year before. By early
November, it was clear that Frazer Hines would be unable to take a major
role in the story because of his obligations to the soap opera
Emmerdale Farm. Sergeant Benton therefore took over Jamie's role --
without the romantic overtones -- though it was hoped for a time that
Hines might still be able to tape a brief cameo appearance for the story's
conclusion. When this proved impossible, it was suggested that Wendy
Padbury might appear as Zoe Heriot. These plans were apparently scuttled
by Pertwee, however, who was concerned that an abundance of old characters
might detract from the show's current protagonists.

Production on “The Black Hole” began on November 6th, with one
day at the Ealing Television Film Studios. This was spent on the fight
between the Third Doctor and Omega's champion, as well as all of
Hartnell's material. To compensate for the actor's failing memory, his
lines were written on cue cards from which he could read as he was filmed.
It transpired that this was Hartnell's final performance; he succumbed to
his health problems on April 23rd, 1975.

William Hartnell's work on November 6th would turn out to
be his final performance

Location work began on November 7th. Three sites in Rickmansworth,
Hertfordshire were visited: the Summerfield Bungalow was Ollis' cottage,
Springwell Reservoir served as the site where the weather balloon touched
down, and Springwell Quarry posed as Omega's world. More filming at the
quarry occurred on the 8th and 9th. November 10th saw cast and crew head
to Halings House in Denham Green, Buckinghamshire, for scenes set outside
UNIT headquarters. Two days of model filming then took place on the 14th
and 15th at the BBC Television Centre Puppet Theatre.

Mayne originally intended to record one episode on each of his four studio
days, but subsequently decided to modify this approach. For the first
studio block, on Monday the 27th and Tuesday the 28th in BBC Television
Centre Studio 1, Mayne taped all the scenes for the first two episodes
(except the part two scenes inside Omega's stronghold) as well as the Time
Lord sequences for the third and fourth installments. These days saw the
debut of the new TARDIS set, designed by Roger Liminton. More faithful to
Peter Brachacki's original design, it replaced the set Tim Gleeson had
conceived for The Time Monster, which had
warped in storage and had met with general disapproval. By now, the
story's title had reverted to The Three Doctors.

The second studio block took place exactly two weeks later, on December
11th and 12th; this time the venue was TC6. Two and a half weeks later,
the first episode of The Three Doctors led off Doctor Who's
tenth season. The story was a milestone in more ways than one. With
Doctor Who's ratings now stable after the ups and down of the late
Sixties, Letts and Dicks had decided to use The Three Doctors as a
vehicle for removing the Earthbound constraints which had been imposed on
the programme for budgetary reasons at the end of the Troughton era. The
Doctor was once again free to travel in all of time and space...

Sources

Doctor Who: The Handbook: The Third Doctor by David J Howe and
Stephen James Walker (1996), Virgin Publishing, ISBN 0 426 20486 7.